Common error when uploading files?

When I run the selected file and press upload image files the following error:

Compilation Error
Description: An error occurred during three days the compilation of a resource required to service this request. Please review the specific error details and modify sau your source code appropriately.

Compiler Error Message: CS0103: The name 'Common' does not exist in the current context

Source Error:

Line 51:
Line 52: / / file name to not be centralized file + extension of the file to upload
Line 53: strFileName = Common.GetFileName () + System.IO.Path.GetExtension (Hinh1.FileName) / / this line error immediately
Line 54:
Line 55: / / Path Large file I save

Common under which this namespace? I have tried namespace
using System.Data.Common;
but not